The present invention relates generally to multi-component spraying systems and, more particularly, to an external mix, air-assisted, airless-atomization, plural component spraying system and method.
Multi-component spraying systems are used in manufacturing plastic articles by applying resinous materials to a mold or preform for an article. In such systems, a liquid resin and a catalyst for the resin are formed into spray particles directed to a substrate where the catalyst and resin react and harden to form the article. In such applications, the resin and catalyst components are preferably mixed together; and the mixture is sprayed onto the substrate. For example, in manufacturing articles with polyester resin, a catalyzing agent for the polyester resin is mixed with the resin; and the resin-catalyst mixture is applied to the substrate. In internal mix systems, the resin and catalyst are mixed within the spraying apparatus; and the mixture is atomized by a spray nozzle and directed onto the substrate. In external mix systems, the resin and catalyst are mixed externally of the apparatus after the resin and catalyst have been atomized. In both external mix and internal mix systems, complete and thorough mixing of the resin and catalyst is important to avoid non-uniform hardening of the resin on the substrate and other undesirable results.
In many spraying systems, large quantities of pressurized air are used to atomize the liquid components. Such systems are expensive to operate and have a number of operational inadequacies. It is expensive to compress air, and the large quantities of compressed air used by existing systems impose a significant operating cost on the system. In addition, the blast of compressed air used to atomize the liquid components carries a significant quantity of spray particles away from the substrate, wastes the expensive resin and catalyst, creates an unclean spray area and sometimes requires overspray collection systems, and contributes to the problem of operating such manufacturing operations safely. Furthermore, the use of large quantities of air during operation of the system can often create an undesirable spread of fumes.
In order to overcome some of the inadequacies attending the use of pressurized air to atomize components dispensed from a spraying apparatus, spraying systems have been developed which incorporate airless atomization techniques.
In prior airless atomization devices, an airless spray nozzle has been used to atomize liquid materials which are pumped at high pressure, that is, pressures generally exceeding 500-600 p.s.i. and more frequently in excess of 800 p.s.i., typical operating pressure being 1000-1500 p.s.i. The most commonly used airless nozzle includes an internal, hemispherical passage termination which is cut through by an external, V-shaped groove to form an elongated, elliptical-like orifice. Liquid material pumped at high pressures through such a spray nozzle is forced by the hemispherical termination of the passageway to converge in its flow at and through the elongated orifice. Because of the converging flow at the orifice, the liquid material is expelled through the orifice into a planar, expanding, fan-like film which breaks into spray particles which are carried by their momentum to the article target.
With viscous fluids, such as the resins used in plural component spraying systems to manufacture plastic articles, high pressures of 1000 to 1500 p.s.i. are required. Such high operating pressures impose a strain on system components reducing their reliability, require generally expensive components in the fluid delivery systems, and contribute to the problem of operating such systems safely. Even at high pressures, however, such fan-like films, because they are formed by the convergence of the fluid, include heavy streams at the edges of the planar, fan-like film which are referred to as "tails". Because of the heavy streamlike flow in the "tails", the spray pattern formed by these edge portions of the expanding, fan-like film includes a disproportionate quantity of resin and produces a non-uniform deposit with stripes when the spray pattern is swept across a substrate by a spray gun operator. The non-uniform deposit and resulting stripes make the bending of deposited material into a film of uniform thickness virtually impossible.
Past efforts to solve the problem of the "tails" attending the use of airless spray nozzles have included the insertion of a "preorifice" immediately behind the elongated, elliptical-shaped orifice to concentrate a greater portion of the flow in the central portion of the fan. Although preorifices are helpful, they are not completely satisfactory, adding another source of clogging to the spray gun and another variable factor to be integrated into system operation.
Compressed air has also been used to solve the problem of tails created by airless spray nozzles. See, for example, U.S. Pat. Nos. 3,202,363; 3,521,824; 3,635,400; 3,843,052; and 4,386,739 and Japanese patent publication No. 57-90762. In plural component spraying systems, compressed air has been used to assist in the atomization of plural component materials as shown, for example, in U.S. Pat. Nos. 2,780,496; 2,864,653; 3,799,403; and 4,618,098 and British patent specification No. 735,983.
External mix plural component systems originally included a plurality of separated spray gun or spray nozzles that were directed to blend their patterns together and to mix thereby resins and their catalysts or hardening agents. See, for example, U.S. Pat. Nos. 3,033,472; 3,399,834; 3,542,296; and 3,788,555. More recently, external mix plural component systems have included the plural spray nozzles in a combined nozzle assembly. See, for example, U.S. Pat. Nos. Des. 252,097; 3,893,621; 4,123,007; and 4,618,098.
In prior art external mix, plural component spraying systems using airless resin nozzles, catalyst has been injected into the resin spray formed from an airless spray nozzle at distances on the order of one inch or more in front of the airless spray nozzle. This downstream location for insertion of the resin spray provided mixing of the catalyst spray particles with resin spray particles which had already been formed from the liquid resin at this location. In such prior plural component systems, resin spray particles are formed within a fraction of an inch of the airless spray nozzle, either under the influence of high hydraulic resin pressures, typically on the order of 1,000 psi, or the combined action of lower hydraulic resin pressures and a plurality of compressed air jets located adjacent the airless resin nozzle and directed at the expanding fan-like resin film. Introduction of the catalyst to the spray an inch or more downstream of the liquid orifice was also necessary to avoid the collection of catalyst on the resin nozzle. In prior external mix spraying systems, catalyst spray particles introduced closely adjacent the spray nozzle frequently accumulated on the resin nozzle. An accumulation of catalyst on the resin nozzle will combine with resin at the resin nozzle orifice and cure the resin, blocking the resin nozzle orifice and, requiring removal of the resin nozzle for cleaning or replacement.
In such prior external mix systems, a substantial flow of air accompanied the rapidly moving resin particles at the downstream location of catalyst injection; and this substantial flow of air was transverse to the direction of the catalyst spray being injected into the spray pattern and made it difficult to inject catalyst particles uniformly into the resin spray. In such prior external mix systems, the catalyst particles were injected into a flow of compressed air by the associated apparatus to blow them into and mix them with the resin spray particles. The flows of air accompanying the formation of the resin particles and used to blow the catalyst particles into the resin spray produced uncontrolled billowing air movements which prevented the fine catalyst particles from being incorporated into the spray pattern and being mixed with the resin particles and deposited on the substrate. More importantly, the air flows associated with such prior external mix systems led to the escape of fine catalyst particles into the surrounding environment, thus presenting cleaning problems and requiring air removal systems.
Furthermore, in such prior external mix systems, it was difficult to obtain desirable spray patterns. The use of the plurality of compressed air jets to assist in atomization of the expanding resin film directly adjacent the liquid orifice of the airless nozzle, where the film had substantial integrity, resulted in a deflection of a portion of this compressed air and contributed to the uncontrolled billowing. This was especially true in systems in which the compressed air jets were directed against airless nozzle itself. The focus of the compressed air jets to assist the atomization of the resin film at the airless resin nozzle made it difficult to effectively use the compressed air from the jets to form resin and catalyst particles into a desirable spray pattern. Furthermore, because of the direction and force required of the compressed air to carry the catalyst particles into the resin spray and to achieve effective mixing of the catalyst particles with resin particles, the compressed air used to entrain the catalyst particles could not be effectively used to provide a satisfactory spray pattern.